Generally injection molding halves for the molding of optical disks provide a cavity located between a movable mold half and a stationary mold half with which a sprue bushing is provided, in order to mold the optical disks by injecting molten resin into the cavity through the sprue bushing. Such apparatus also provides a punch associated with the movable mold half concentric to the sprue bushings to sever a central aperture of the molded disks by forward motion of the punch.
As such injection molding halves for molding optical disks in the prior art are composed of but a single cavity, there are limits in improvements of productivity possible. But there exists difficulty in carrying out injection molding with a plurality of cavities formed between the stationary mold half and movable mold half, that is, molten resin has to flow into plural cavities through plural sprue bushings which are provided in the stationary mold half and connected to a passage provided with the stationary mold base plate, and moreover each of central apertures of a plurality of the disks molded in the respective cavities has to be severed by forwarding of each punch provided with the movable mold half.
In such mold halves, the construction becomes overly complicated as each front portion of the sprue bushings is forwarded to a predetermined position while injecting molten resin in order to make the gate annular in the center of the each cavity and is retracted according to the forwarding motion of each punch when severing the central aperture of the disk.